Repository pattern - my preferred implementation...

Okay it's nothing new and not even original but I wouldn't to get down my currently preferred implementation of the repository pattern. I suppose this was prompted by a blog by Jimmy Bogard and Oren's statement a couple of months ago the repository pattern may be near then end of it's life.

I still think in the .Net world they have great relevance as most .Net devs can't organise code for toffee and when you try and introduce layering into an application the use of an explicit repository layer is the first layer they seem to understand.

So here is my current repository flavour - strawberry with a twist of lemon...

 public sealed class Repository<T1, T2> : IRepository<T1, T2> where T1 : IEntity<T2>
    {
        private readonly ISession _session;
        private readonly string _traceType;

        public Repository(IProvideSessions sessionFactory)
        {
            _traceType = string.Format("Repository<{0}, {1}>: ", typeof(T1).Name, typeof(T2).Name);
            _session = sessionFactory.GetSession();
        }

        public T1 Get(T2 id)
        {
            Trace.WriteLine(string.Format("{0} Get - '{1}'.", _traceType, id));
            return _session.Get<T1>(id);
        }

        public void Save(T1 instance)
        {
            Trace.WriteLine(string.Format("{0} Save - '{1}'.", _traceType, instance.Id));
            _session.Save(instance);
        }

        public void Delete(T1 instance)
        {
            Trace.WriteLine(string.Format("{0} Delete - '{1}'.", _traceType, instance.Id));
            _session.Delete(instance);
        }

        public T1 FindOne(IFindable findable)
        {
            Trace.WriteLine(string.Format("{0} FindOne, - '{1}'.", _traceType, findable.GetType().Name));
            var criteria = findable.BuildCriteria();
            return criteria.UniqueResult<T1>();
        }

        public IList<T1> FindAll(IFindable findable)
        {
            Trace.WriteLine(string.Format("{0} FindAll, - '{1}'.", _traceType, findable.GetType().Name));
            var criteria = findable.BuildCriteria();
            return criteria.List<T1>();
        }
    }

One of the first things to notice is the use of two generic types T1 & T2 - I'm not great at naming generic parameters so they never got better names. Hopefully it's obvious but T1 is the domain entity and T2 represents the 'Id' column for the entity (All DDD entities have Id's).

The other important feature is the 'FindOne' & 'FindAll' methods they take an implementation of the interface IFindable which performs the magic. Now this is where the generic repository starts to have a 'leaky abstraction' and this happens to be exposing nHibernate's ICriteria interface vai the IFindable interface. The implementation of the IFindable is responible for the creation of the NH criteria and returns this when requested, this is then executed by the repository and volia the results are returned.

So my current repository pattern is designed to be used with NH, but if a client dictates I can't use NH then I will modify the IFindable interface accordingly or I will terminate the contract depending on how I'm feeling :)

 public interface IFindable    {        ICriteria BuildCriteria();    }

An example of this could the a Findable class that returns an NH criteria that will return bank customers with a balance greater than million - Millionaires!

public sealed class FindValuedCustomers : IFindable
    {
        private readonly ISession _session;

        public FindValuedCustomers(IProvideSessions sessionFactory)
        {
            _session = sessionFactory.GetSession();
        }

        public ICriteria BuildCriteria()
        {
            return _session.CreateCriteria(typeof(Account)).Add(Property.ForName("CurrentBalance").Gt(1000000));
        }
    }

Now I can see how Oren goes from this to the idea of just using NH Session anywhere in the code you previously used a Repository but I do think there is still some requirement to provide a layer & abstraction for testing purposes.

One other thing to note, I do believe the Repository pattern has valid uses outside of DDD, as Eric has stated most of the common patterns in DDD existed before the book.



Awkward Coder